1. Field of the Invention
The present invention relates to atmospheric wind measuring devices and, more particularly, to an atmospheric remote sensing instrument that takes wind measurements by sending out acoustic pulses and then measuring acoustic energy reflected back to the instrument by the atmosphere.
2. Description of the Prior Art
Measuring wind changes with altitude is necessary in order to provide the input data that meteorological models require so as to produce the most accurate results. Weather forecasters have traditionally used weather balloons, and more recently weather satellites, to measure wind changes at altitudes of 50 kilometers or more. In the lower atmosphere, however, balloons and satellites have difficulty providing the kind of high resolution, continuous measurements that are needed in meteorological modeling for air quality decisions such as permit approval or photochemical control strategy development. A practical answer to this measurement need, now in common use, is to "listen" to the winds in the first few hundreds of meters above the ground using an instrument called a SODAR.
The term SODAR is an acronym for "Sound Detection And Ranging". It refers to an atmospheric remote sensing instrument that takes measurements in vertical profiles directly above the instrument's outdoor antenna system. These vertical profiles are commonly called soundings, and a SODAR is, therefore, commonly called an acoustic sounder. A SODAR is a remote sensing instrument because it measures the atmosphere above its antenna system remotely, which means that the instrument does not directly touch the air being measured. It takes the measurements by sending acoustic pulses upward and then measuring the acoustic energy reflected back to the instrument by the atmosphere. Unlike a meteorological instrument that takes point measurements wherever it is positioned, a SODAR remotely measures a volume of air above its antenna system and averages over the volume in height intervals to provide a single reading for each height interval.
Although several types of SODAR instruments exist, the present invention is concerned with a so called monostatic SODAR. A monostatic SODAR, such as that disclosed in U.S. Pat. No. 4,558,594, uses a single antenna system to both transmit acoustic signals upward into the atmosphere and to measure the reflection of those signals back from small scale turbulence (caused by small scale thermal gradients) in the air. While the characteristics of the antenna system can vary significantly by manufacturer, the basic technique requires that measurements be made sequentially and repetitively along three beam paths, one of which is vertical and the other two slightly tilted off vertical and orthogonal to each other (i.e., the two tilted beams might point southeast and northeast, west and north, northwest and southwest, etc.). Standard measurements are made by transmitting a pulse along the first beam path, then waiting a few seconds for all reflected energy from the atmosphere to be received back at the antenna, then transmitting on the second beam path and waiting for all reflected energy, then transmitting on the third beam path and waiting for all reflected energy, then continuing to repeat this "ping" cycle continuously to accumulate measurements for averaging during automated data processing. Alternative SODAR configurations use more than three beam paths to produce more reliable and accurate wind measurements. The data from the extra beam directions is used for cross-checking, to detect and compensate for erroneous radial wind samples. For example, a SODAR configuration using five beams could use one vertical beam path and four beams tilted slightly of vertical and orthogonal to each other (e.g. the tilted beams might point east, north, west, and south). Similar to the manner described for the three beam configuration, pulses would be transmitted along each of the five beams in sequence and the echo signals would be analyzed so as to determine the radial winds along each of the beam axes. Three of the five measured radial winds (from the vertical, north, and south beams) would contain information about the north/south component of the wind, which can be computed from any two of these three values. The extra measurement is available for quality assurance computations, to determine whether the set of three radial winds is consistent with a set of north/south and vertical wind component values. In a similar manner, another subset of measured radial winds (from the vertical, east, and west beams) would contain redundant data for computing a verified east/west wind component. Signal processing algorithms can be used to reliably determine which radial wind samples are erroneous.
The approach just described may be applied to any configuration with four or more beams, as long as three independent beam directions are present. It is not necessary for subsets of three beam axes to be coplanar, as in the five beam example above. If that is the case, however, the calculations are simpler.
SODARs are commonly called "Doppler SODARs" because they use the frequency difference (called "Doppler shift") between the transmitted and the reflected acoustic energy to determine the movement of air that reflected the acoustic energy. The frequency shift from each beam path is converted into a radial wind along that path, and the radial winds from the respective beam paths are then combined (using mathematical computations) to produce horizontal wind direction and speed at designated height intervals in the vertical profile above the antenna system. The resultant horizontal wind direction and speed value for each vertical interval represents an average for the volume measured and over the time span designated by the operator. The size of the volume measured depends on the characteristics of the beams used and on the depth of the height intervals set by the operator. The SODAR assigns heights to the data according to length of time it took for the increments of the reflected acoustic energy to be received back at the antenna.
Using the wind data, some SODARs provide derivative information such as wind components (u,v,w), standard deviations (.sigma.w,.sigma..theta.,.sigma..phi.), and stability class estimates (Showalter Classes A-F). In addition to analyzing for Doppler shift to estimate winds, SODARs also commonly record the strength of the reflected acoustic energy, called "backscatter." When displayed in time-height cross section, "backscatter" data recorded by the SODAR show atmospheric thermal structure patterns that can be interpreted by either automated software algorithms or a meteorologist to provide estimates of mixing heights, a feature particularly important to air pollution applications.
The primary factors influencing SODAR performance are atmospheric conditions, siting conditions, and the configuration of the SODAR system being used. Atmospheric and siting conditions dictate how much reflected atmospheric "signal" a SODAR will be able to correctly recognize. Since acoustic energy attenuates rapidly with distance, the height measurement capability of a SODAR will improve and degrade over time as the capacity of the atmosphere to strongly reflect acoustic signals changes. Also, since the SODAR will "hear" noises from both the atmosphere (e.g., rain) and other sources (e.g., automobile traffic) within its transmit frequency range, the height capability of a SODAR will vary according to the strength of the background noise of the atmosphere and siting environment in which it is operating. The adverse impact of background noise can be reduced somewhat by use of an acoustic cuff around the antenna array. Also important in the siting environment is the presence or absence of nearby structures that may reflect acoustic energy back into the antenna system and interfere with its operation. An operator can usually control the height sensing capability of a SODAR to a certain extent according to the configuration of the particular SODAR system being used. The most important factor in this area is the "pulse length," which helps control how much acoustic energy is transmitted. The longer the pulse length set, the more energy transmitted. Longer pulse lengths help reach to greater measurement heights, but tend to also raise the minimum altitude that can be accurately measured and the minimum vertical interval that can be measured. An operator can usually also influence sensing height capability by the length of the time averaging period used for data measurements. For example, setting a particular SODAR system to record 15 minute average winds will normally allow greater sensing altitude than 5 minute average winds because it increases the probability that usable reflected energy samples will be recorded at the upper sensing altitudes during the time averaging period (simply because the averaging period is longer). Also longer averaging times can benefit signal processing by tending to improve both data quality and altitude coverage. An operator can usually also set the receive gain according to the sensing height desired. The maximum gain setting would normally be used when the emphasis is on sensing as high as possible. Conversely, reducing the gain can be helpful when the emphasis is on the lower sensing altitudes, as a gain setting too high can contribute to receiver saturation causing data loss in the lower gates. Regarding height sensing capability in general, the condition of the atmosphere is the single greatest factor in determining to what height data can be collected. Limiting atmospheric conditions include wind speeds and the presence or absence of small-scale thermal gradients. Wind speeds are important in that strong winds tend to blow the transmitted acoustic beams laterally and the reflected acoustic energy from greater heights may therefore also be laterally displayed and not received back at the antenna. Small-scale thermal gradients are important, because a monostatic Doppler SODAR relies on the presence of these gradients to reflect the acoustic energy back to the antenna. This means that when the atmosphere is well mixed, sensing conditions are relatively poor. Well mixed conditions that occur throughout the sensing range of a SODAR result in only the lower gates being measurable, while well mixed conditions that occur in layers within the sensing range of a SODAR result in those layers not being measurable, whereas the lessor mixed layers above and below are measurable.